In the related art, a semiconductor storage apparatus including memory cells that hold amounts of charges corresponding to data values is widely known. A data value stored in a memory cell is determined on the basis of comparison of a threshold voltage corresponding to an amount of charges held in the memory cell with a predetermined reading voltage. An amount of charges held in a memory cell varies with time and for each access. The data value varies with the variation in the amount of charges. The varied data value is detected as an error. To decrease the number of data values which are detected as errors, a technique of shifting a reading voltage with the variation in the amount of charges has been developed.